1. Field Of The Invention
This invention relates to systems and circuits that will enhance the effect of stereo sound. More particularly this invention relates to a preprocessor that will modify the electrical signals of the stereo sound such that the sound broadcast from the speakers will mimic the characteristics of sound broadcast in an acoustically ideal room.
2. Description of Related Art
Stereo sound imaging perceived by a listener using a traditional two-channel stereo sound reproduction system depends on the acoustic properties of the listening environment, i.e. the acoustic properties of the room. Although the most direct way for improving the perceived sound imaging is to improve the acoustic properties of the room, i.e. to alter the geometry of the room, ceiling heights, wall separations, angles of tilts for ceilings and walls, and the acoustic properties of the materials used, such an approach is definitely costly. An alternative approach is to place an electronic preprocessor before the speakers to preprocess the signals delivered to the speakers so that the signals perceived by the listener(s) mimics as closely as possible the signals perceived in an ideal acoustic room.
Consider a scenario comprising of an acoustic radiator and a sensor in an enclosed room as shown in FIG. 1. Acoustic waves radiated from the radiator propagate to the sensor through a multipath channel. The followings are some examples of the paths. Path #1 is the direct path. Path #2 is a single reflection path. Path #3 and path #4 are multiple reflection paths.
The transmission characteristics from the signal source to the sensor may be described by the Laplace transform transfer function, H(s), given by ##EQU1##
where:
N is the order of the transfer function. PA1 A is the transfer function of the direct acoustic path between a right-channel sound source and a listener's ear. PA1 B is the transfer function of the direct acoustic path between a left-channel sound source and listener's ear.
H(s) may be obtained from the autocorrelation and cross correlation functions of the signal radiated from the radiator and the signal received by the sensor.
A listener in a room with two speakers as shown in FIG. 2 is an example of a two-radiator two-sensor system. In FIG. 2, the two speakers are the two radiators and the two ears of the listener are the two sensors. The transmission characteristics from the right speaker Speaker-R to the left ear Ear-L and the right ear Ear-R may be represented by the transfer function H.sub.RL (s) and H.sub.RR (s), respectively. Similarly, the transmission characteristics from the left speaker Speaker-L to the left ear Ear-L and the right ear Ear-R may be represented by the transfer function H.sub.LL (s) and H.sub.LR (s), respectively.
U.S. Pat. No. 5,440,638 (Lowe et al.) describes a preprocessor to enhance the sound field in a stereo reproduction system. A portion of the audio information that is common or substantially common to both the left and right stereo input signals is removed. The remaining components are processed in left and right sound placement filters. The outputs of the left and right placement filters are then added respectively to the right and left stereo input signals. This will produce an enhanced sound field at the stereo outputs. The input signal not processed will be delayed to maintain coherency.
U.S. Pat. No. 4,980,914 (Kunugi et al.) discloses a sound field correcting system. The sound field correcting system will correct multipath frequency characteristic distortion in an acoustic reproduction system. The level and delay of an original signal are adjusted and superposed on the original signal so as to obtain a signal which, when reproduced by a loudspeaker, yields a sound pattern at a listening point having a flat frequency characteristic. The delay adjustment is effected in accordance with a difference between the travel distances of direct and reflected sound waves to the listening point.
U.S. Pat. No. 4,355,203 (Cohen) describes a stereo enhancement system that utilizes a difference signal. The difference signal is derived from the left and right stereo channels in which the difference signal is delayed, amplified, and then added into the appropriate channels to cancel left/right speaker mixing at the listeners ear. This is to improve stereo separation without center region distortion.
U.S. Pat. No. 4,209,665 (Iwahara) discloses a signal translator that include right- and left-channel translating networks. The right- and left-channel translating networks are constructed to have a transfer function: ##EQU2##
where:
Through the right- and left-channel components of spatially correlated audio signals under go transformation of: ##EQU3##
When binaural signals are applied to the translating networks, the translated output signals are applied to a pair of loudspeakers in a listening room. The acoustic direct paths and crosstalk paths transform the signal so that the impinging sound at the listener's ear is distortion free.